The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Nuclear reactors, such as boiling water reactors, generally include a reactor core comprised of many fuel bundles through which, and around which, a liquid moderator, e.g., liquid water, flows. Nuclear reactions within the fuel bundles generate heat used to convert the moderator to steam as the moderator passes through the core. The steam is then used to generate electrical power. Each of the fuel bundles typically includes a plurality of sealed and vertically upstanding fuel rods housed within an elongate tubular channel. Within the channel, the fuel rods of each fuel bundle are held in a spaced apart configuration by two or more spacer grids comprised of a plurality of interconnected spacers that form a plurality of rows and columns of open cells. Each cell has a respective one of the otherwise long and flexible fuel rods extending therethrough and serves to prevent the fuel rods from coming into abrading contact one with another under the dynamics of moderator flow within the reactor. The spacers additionally maintain the designed fuel rod to fuel rod spacing for optimum nuclear performance. The spacer grids typically include a perimeter band that provides an outer defining envelope for the spacer cells and the fuel rods placed therein.
Known spacer grid designs are typically sized such that a small gap exists between the perimeter band and the walls of the channel that allows the grouped fuel rods, i.e., the fuel rods retained within the spacer grids, to be more easily inserted into the respective channels. However, this gap allows movement of the grouped fuel rods within the respective channels. For example, when the fuel bundles are placed within a reactor, such movement can be caused by many forces within the reactor, such as the moderator flow. Movement of the grouped fuel rods within a reactor can cause some of the peripheral, i.e., outermost, fuel rods of the group to move towards the channel walls while other peripheral fuel rods are moved away from the channel walls. When any of the peripheral fuel rods move toward one or more of the channel walls, the flow of the moderating coolant is inhibited at these highly reactive fuel rods. Inhibition of coolant flow causes critical power losses at these peripheral fuel rods, especially at the fuel rods adjacent the corners of the channel. As a result, the entire fuel bundle must be limited in its performance so that these critical power limits of the peripheral fuel rods are not exceeded.
Another example of the grouped fuel rods moving within the respective channel occurs during shipping of the fuel bundles. During shipping, the gap between the channel and the spacer grids can allow the grouped fuel rods to move, or ‘rattle’, within the channels and cause structural damage to the spacers grids and fretting damage to the fuel rods.
A further disadvantage of known spacer grids, particularly the perimeter band, is that the structural design of such spacer grids can inhibit the flow of coolant between the channel walls and the peripheral fuel rods, which can limit the energy generating potential of the peripheral fuel rods. Further yet, the perimeter bands do little or nothing to ‘strip’ coolant from the channel walls so that the coolant is utilized to cool the grouped fuel rods.